How to add these stripes to my fender?
As you said you can simply unwrap:


Create your stripes with a transparent background and save it as PNG.
Make an unwrap of the part of the mesh that will display the stripes and give this UVmap the name you want.
Duplicate your basic car material and assign this copy to this part of the mesh.
In this material, mix your basic node(s) with the one(s) you specifically want to give to the stripes.
Add an Image Texture node, load the PNG, and use the Alpha output as the factor of a Mix Shader.
In the Image Texture node, use Clip instead of Repeat so that the stripes won't repeat.
Plug an Input > UV Map node in the Vector of the Image Texture node and load the UVmap you've created in this UV Map node.
Now adjust the UV in the UV/Image Editor until the stripes is correctly displayed.